Internal reflectance spectroscopy (IRS) has been accepted as an analytical technique for the analysis of solids and liquids. As a production quality control or a quality assurance method, the IRS analysis of liquids, pastes, mulls, etc. has become quite wide spread. The popularity of using IRS for liquid samples has increased with the acceptance of the Spectra-Tech Circle.RTM. cell and horizontal ATR (Contactor.TM.) accessories. Spectra-Tech's Circle.RTM. cell is disclosed in U.S. Pat. No. 4,595,833. Spectra-Tech's horizontal ATR accessory is disclosed in U.S. Pat. No. 4,730,882.
The success of IRS analysis of liquids can, in large part, be attributed to the accuracy, repeatability, precision and reliability of the measurement technique. In addition, for liquids and liquid like materials (i.e., free-flowing "fluids"), IRS is a simple, easy to use method of analysis normally incorporating a multiple bounce radiant energy path. In most cases, however, modest amounts of liquids are required to completely wet the IREs currently in use, which typically have cylindrical contacting surfaces or larger than required flat contacting areas. Specifically, for detection of HPLC, SFC or other eluents, current IRE techniques require the presence of relatively large sample quantities.
For solids, highly reliable, repeatable and photometrically accurate results are not as easily achieved with currently existing apparatus, except for those solids with highly regular, special property surfaces. Since the IRS technique depends upon the electromagnetic coupling of energy within the IRE to the sample in the immediate vicinity (less than 10 microns) of the IRE sample contacting surface, the surface morphology of the sample greatly influences the extent of coupling and the resulting measurements.
Manufacturers of infrared sampling apparatus, as well as users of the IRS technique, have proposed several structures or procedures over the past 20 to 30 years to better insure proper electromagnetic coupling with solid samples. While these techniques may have provided limited improvements in electromagnetic coupling, no general structure or process has gained any widespread acceptance in the industry, especially when quantitative data on solid samples is required.
The following four examples of structure and/or processes to improve electromagnetic coupling of the IRE crystal to the sample are illustrative of some of the prior art techniques. First, the sample can be forced by the application of an evenly distributed pressure against a surface of the IRE. The pressure can be applied pneumatically, hydraulically or mechanically. Second, a high index of refraction oil can be used as a coupling fluid between the sample and the IRE to better match refractive indices through that oil rather than through air. Third, the sample may be heated to soften or recast the material (usually a polymer) onto the sampling surface of the IRE. Fourth, the sample may be dissolved with a solvent so that the resulting liquid can be analyzed and/or an evaporated thin film on the IRE can be analyzed. These IRS solid sampling techniques may be used with many of the known or commercially available IREs.
The books Internal Reflectance Spectroscopy (1967, John Wiley & Son, Inc.) and IRS Review and Supplement (1985, Marcel Dekker, Inc.) written by Harrick disclosed IRE's for the analysis of materials by using the attenuated total reflectance (ATR) phenomena. These IRE's are generally classified as single internal reflection elements (SIRE) or multiple internal reflection elements (MIRE). The acronym IRE as used herein means both SIRE's and MIRE's.
The known IRE shapes used with liquids include trapezoidally shaped prisms, cylindrical rods and hemispheric domes. IRE's used for IRS analysis of solids include fixed angle prisms and variable angle IRE's including hemicylinders, microhemicylinders, hemispheres with flat bottoms and hemispheres with truncated conical bottoms. On page 98 of the Internal Reflectance Spectroscopy book, Harrick illustrates the hemicylinder IRE with flat bottom and the hemispherical IRE with the truncated conical bottom.
In Harrick's noted illustrations, the bottom or sample contacting surface of the IRE is shown to be flat and the sample is shown to be flat. The commercially available IREs for solid sampling are all believed to have flat, sample contacting surface areas of sizes well exceeding 250 microns in diameter.
Harrick categorizes the hemicylinder and hemisphere as variable angle IRE's. Harrick specifies that the angle of incidence is not well defined in these variable IRE's, thereby suggesting that the use of such elements will yield unreliable or non-repeatable results. On page 99 of the Internal Reflectance Spectroscopy book, Harrick explains that a modified hemisphere with a truncated conical flat bottom was used by Fahrenfort to facilitate contacting sample material.